In the prior art, each glass top range element has a high temperature limit that operates according to the rod expansion principle. The high temperature limit consists of a quartz tube with an expansion or sensing rod inside and a dust protected ceramic head with a built-in snap action mechanism to make and break two sets of contacts. The first set of contacts controls a common, remote warning light circuit, and the second set of contacts controls the power to the heating element.
With present day technology, the electric range has a glass/ceramic cooking top upon which rests the cooking utensils. One or more electric resistance heating elements are mounted on the underside of the glass/ceramic top. There is a high temperature limit mounted on each heating element. When a heating element is energized, the sensing rod in the high temperature limit expands as the temperature rises and closes the first set of contacts, thereby allowing current to flow to a single common centrally located remote warning light. This light informs the user that a section of the glass is hot, but not which section. This could result in an injury to the end user of the product.
A prior art circuit and switch are depicted in FIG. 1 and a description of a sequence of operation for such a standard limit switch is as follows.
In the first step, an infinite switch 1 is manually turned on. With the switch on, current flows from the main power supply L1 through the infinite switch 1 to the common remote power-on indicator light 3, and through the high temperature limit-hot light switch unit 4. The unit 4 includes a limit switch 5 that employs a normally closed set of contacts 9 and 11, a hot light switch 17, that employs a normally open set of contacts 19 and 21, and a sensing rod 15 positioned so as to monitor the temperature of the heating element 13 and the glass (not shown). The current flows through the closed set of contacts 9 and 11 to the heating element 13, and back to the main power supply L2.
In a second step, the heating element 13 starts to heat as a result of the infinite switch 1 being turned on. As the temperature at the heating element 13 rises, the sensing rod 15 in the unit 4 starts to expand. At a predetermined point, the sensing rod 15 expands and closes the hot light switch 17. When the switch 17 closes as a result of the expansion of the rod 15, contacts 19 and 21 are connected completing the circuit and the common remote warning light 23 on a control panel is energized, thereby indicating that a hot surface exists on the range top.
As the heating element 13 continues to heat, the sensing rod 15 in the unit 4 expands further and breaks the contact between 9 and 11, thereby disconnecting the power to the heating element 13. As the heating element 13 cools, the sensing rod 15 cools and its length decreases to close switch 5 and make connection again between the contacts 9 and 11. Continued heating and cooling affects the length of the sensing rod 15 and the heating element 13 cycles off and on as switch 5 opens and closes. This cycling action thereby limits the heating element's output to a safe level.
In a third step, the heating element 13 is turned off by turning the infinite switch 1 to the off position. The power to the heater is then interrupted. However, the hot light 23 remains on until the sensing rod 15 in the unit 4 shortens sufficiently to open the switch 17, indicating that the glass or cooking surface has cooled to a point below a predetermined temperature deemed safe for the end user to touch without injury.
The circuit depicted above does not tell a user as to which range top surface is at a dangerous temperature. In light of this deficiency in the prior art, a need has developed to provide an improved limit switch and circuit for heating elements, particularly those associated with cooking stoves using electrical heat for cooking. The present invention solves this need by providing a switch that both indicates that power is supplied to a given heating element and that that particular element is unsafe to touch since it is too hot, regardless of whether power is being supplied to the element.